1. Field
The present invention relates to certain substituted pyridine and pyrazine compounds and derivatives of such compounds; pharmaceutical compositions containing them methods of making them; and their use in various methods, including the inhibition of PDE4 enzymes; detection and imaging techniques; enhancing neuronal plasticity; treating neurological disorders, including psychiatric, neurodegenerative, cerebrovascular, and cognitive disorders; providing neuroprotection; enhancing the efficiency of cognitive and motor training; facilitating neurorecovery and neurorehabilitation; and treating peripheral disorders, including inflammatory and renal disorders.
2. Description of the Related Technology
The mammalian phosphodiesterases (PDEs) are a group of closely related enzymes divided into 11 families (PDE1-11) based on substrate specificity, inhibitor sensitivity and more recently, on sequence homology. The 11 families are coded by 21 genes, providing several of the families with multiple members. All mammalian PDEs share a conserved catalytic domain located in the COOH-terminal portion of the protein. In GAF-containing PDEs, one or both GAFs can provide dimerization contacts. In addition, one of the GAFs in each of these proteins provides for allosteric cGMP binding (PDE2, PDE5, PDE6, PDE11), allosteric cAMP binding (PDE10), and regulation of catalytic site functions (PDE2, PDE5, PDE6). The other families of PDEs have unique complements of various subdomains (UCR, NHR, PAS, membrane association) that contribute to regulation of activity. PDEs 1, 2, 3, and 4 are expressed in many tissues, whereas others are more restricted. In most cells, PDE3 and PDE4 provide the major portion of cAMP-hydrolyzing activity (Francis, Physiological Reviews, 2011, 91, 651-690).
The PDE4 family includes four isoforms (PDE4A, B, C and D) with more than 20 splice variants, making it one of the largest PDE subfamilies (Bender and Beavo, Pharmacol. Rev., 2006, 58 (3), 488-520). PDE4 enzymes hydrolyze cAMP with a substrate apparent Km of 1-5 uM for cAMP. The PDE4 enzyme is reported to be regulated by two upstream conserved region (UCR) domains. Depending on differential RNA splicing, PDE4 variants can be distinguished into two major subgroups: long and short forms (Conti et al., J Biol. Chem., 2003, 278, 5493-5496). Nine splice variants have been reported. PDE4D1, 4D2 and 4D6 all are shorter forms lacking UCRs. PDE4D3, 4D4, 4D5, 4D7, 4D8 and 4D9 are longer forms that contain both UCRs and N terminal domains important for their subcellular localization (Bender and Beavo, 2006). Long form PDE4D3 activity is increased by PKA phosphorylation via Ser54 in the N-terminal UCR1 (Alvarez et al., Mol. Pharmacol., 1995, 48, 616-622; Sette et al., J Biol. Chem., 1996, 271, 16526-16534). Conversely, Erk2 phosphorylation of Ser597 in the C-terminus of PDE4D3 causes a reduction in catalytic activity. One or several PDE4D isoforms are expressed throughout most tissues tested, including cortex, hippocampus, cerebellum, heart, liver, kidney, lung and testis (Richter et al., Biochem. J., 2005, 388, 803-811). The localization and regulation of PDE4D isoforms is thought to allow for tight and local regulation of cAMP levels, possibly limiting signal propagation in specific subcellular compartments.
Numerous studies have highlighted a role for PDEs generally, and PDE4 in particular, in modulating intracellular signaling pathways that regulate many physiological processes, including those underling neural plasticity, cognition, and memory. In particular, PDEs play an important role in intracellular signal transduction pathways involving the second messengers. cAMP and cGMP. These cyclic nucleotides function as ubiquitous intracellular signaling molecules in all mammalian cells. PDE enzymes hydrolyze cAMP and cGMP by breaking phosphodiester bonds to form the corresponding monophosphates (Bender and Beavo, Pharmacol. Rev., 2006, 58 (3), 488-520). PDE activities are modulated in coordination with adenylyl cyclase (AC) and guanylyl cyclase (GC) activities through direct effectors and feedback pathways, thereby maintaining cAMP and cGMP levels within optimum ranges for responsiveness to signals. The ability of extracellular signals to modulate the intracellular concentration of cyclic nucleotides allows cells to respond to external stimuli across the boundary of the cell membrane.
The cyclic nucleotide signaling cascades have been adapted to respond to a host of transduction systems including G-protein coupled receptors (GPCRs) and voltage and ligand gated ion channels. Cyclic nucleotides transmit their signal in the cell through a variant of tertiary elements. The best described of these are cAMP dependent protein kinase (PKA) and cGMP dependent protein kinase (PKG). The binding of the cyclic nucleotide to each enzyme enables the phosphorylation of downstream enzymes and proteins functioning as effectors or additional elements in the signaling cascade. Of particular importance to memory formation is cAMP activation of PKA, which phosphorylates cAMP response element-binding protein (CREB). pCREB is an activated transcription factor, which binds to specific DNA loci and initiates transcription of multiple genes involved in neuronal plasticity. Both in vitro and in vivo studies have associated alterations in cyclic nucleotide concentrations with biochemical and physiological process linked to cognitive function (Kelly and Brandon, Progress in Brain Research, 2009, 179, 67-73; Schmidt, Current Topics in Medicinal Chemistry, 2010, 10, 222-230). Signal intensity and the levels of coincident activity at a synapse are established variables that can result in potentiation of transmission at a particular synapse. Long term potentiation (LTP) is the best described of these processes and is known to be modulated by both the cAMP and cGMP signaling cascades.
Focus on the role of PDE4 in memory stems from the discovery of the PDE4-like Drosophila learning mutant dunce (dnc gene), a cyclic nucleotide phosphodiesterase of the PDE4 subtype (Yun and Davis, Nucleic Acids Research, 1989, 17(20), 8313-8326). The dnc mutant flies are defective in acquisition and/or short-term memory when tested in several different olfactory associative learning situations, with negative (Dudai et al., Proc Natl Acad. Sci., 1976, 73(5), 1684-1688; Dudai Y., Proc Natl Acad. Sci., 1983, 80(17), 5445-5448; Tully and Quinn, Journal of Comparative Physiology, 1985, 157(2), 263-77) or positive reinforcement (Tempel et al., Proc Natl Acad. Sci., 1983, 80(5), 1482-1486). In mammals, PDE4D knockout animals display decreased immobility in the antidepressant tail-suspension and forced swim test models (Zhang et al., Neuropsychopharmacology, 2002, 27(4), 587-595), enhanced in vitro LTP in hippocampal CA1 slices (Rutten et al., Eur. J. Neurosci., 2008, 28(3), 625-632), and enhanced memory in radial maze, object recognition, and Morris water maze tasks (Li et al., J. Neurosci., 2011, 31, 172-183).
Such observations highlight the interest in PDE-inhibition, including PDE4-inhibition, as a therapeutic target for numerous disorders and in cognitive enhancement. For example, by increasing cAMP levels, such inhibitors may be useful in treating cognitive deterioration in neurodegenerative disorders such Parkinson's Disease and Alzheimer's Disease, as well as generally improving cognition in normal, diseased, and aging subjects. Various small-molecule PDE4 enzyme inhibitors have been reported e.g., Aza-bridged bicycles (DeCODE Genetics; Intl. Pat. Appl. Publ. WO 2010059836, May 27, 2010); N-substituted anilines (Memory Pharmaceuticals Corporation; Intl. Pat. Appl. Publ. WO 2010003084, Jan. 7, 2010); Biaryls (DeCODE Genetics; Intl. Pat. Appl. Publ. WO 2009067600, May 28, 2009, WO 2009067621, May 28, 2009); Benzothiazoles and benzoxazoles (DeCODE Genetics; U.S. Pat. Appl. Publ. US 20090130076, May 21, 2009); Catechols (DeCODE Genetics; U.S. Pat. Appl. Publ. US 20090131530, May 21, 2009), Pteridines (Boehringer Ingelheim International G.m.b.H.; U.S. Pat. No. 7,674,788, Nov. 29, 2007); Heteroaryl pyrazoles (Memory Pharmaceuticals Corporation; Intl. Pat. Appl. Publ. WO 2007123953, Nov. 1, 2007); Naphthyridines (Glaxo Group Limited; Intl. Pat. Appl. Publ. WO 2006053784, May 26, 2006); Piperazinyldihydrothienopyrimidines (Boehringer Ingelheim International G.m.b.H.; EP Pat. 1,874,781, Jun. 24, 2009); Nicotinamide derivatives (Pfizer; U.S. Pat. Appl. Publ. US 20050020587, Jan. 27, 2005); Heteroarylmethyl phenyl amines (Memory Pharmaceuticals Corporation; U.S. Pat. No. 7,087,625, Aug. 8, 2006); Naphthyridines (Novartis AG; EP Pat. 1,443,925, Dec. 26, 2007; U.S. Pat. No. 7,468,370, Dec. 23, 2008).
However, PDE4 inhibitors have generally been associated with numerous side effects—most notably emesis—that have typically limited their usefulness and tolerability (e.g., Giembycz, Curr. Opin. Pharm. 2005, 5, 238-244). It is therefore desirable to develop improved PDE4 inhibitors showing higher potency, greater specificity, and better side effect profiles. The present invention meets these and other needs in the art by disclosing substituted pyridine and pyrazine compounds as potent and well-tolerated PDE4 inhibitors.